The invention relates to magnetic resonance ("MR") imaging, and more particularly relates to magnetic resonance imaging of body parts which are moved as a patient breathes.
It is known that nuclear (particularly hydrogen) spins of a test object can be deflected from a preferential direction which is established by a primary magnetic field. This deflection takes place by a radio-frequency excitation pulse, and only after a certain interval following the excitation pulse will these spins re-orient themselves in the preferential direction. During this interval the spins precess with a frequency which depends on the intensity of the magnetic primary field. The precession can be detected with an RF antenna and corresponding receiver circuits. If a first field gradient is superimposed on the homogeneous magnetic primary field, so that the magnetic field distribution varies spatially, locating of the spins in a first spatial direction, namely the gradient direction, is possible by measuring the respective measured frequency and correlating that frequency with the corresponding position.
It is also known that by a brief application of a second gradient field which is orthogonal to the first gradient, a location coding along a second space axis can be carried out before the signals in the first gradient are read, and that in this manner an MR image of a slice of a test object can be produced. The excitation in a slide of the test object is brought about by the fact that the magnetic primary field is influenced by an additional third field gradient perpendicular to the first and second gradients in such a way that only in this slice does an excitation of the nucleii take place. This is possible because the excitation occurs only with a frequency which is strictly correlated to the magnetic field in the desired slice. This method is described e.g. in German Offenlegungschrift No. 26 11 497.
Because the recording time required for MR imaging is relatively long, image disturbances result through the movement of the heart and lungs when these organs are imaged.
Therefore, heart- and respiration-gated image data acquisition has been used for MR scanning. A method of this kind is described in the publication "Magnetic Resonance Imaging with Respiratory Gating" by Richard L. Ehman et al, AJR: 143, Dec. 1984, pp. 1175-1182. There, a method is disclosed in which the radio-frequency signal is emitted at regular intervals; in accordance with these intervals, spin echoes are generated at regular intervals. However, not all spin echo signals are utilized in the image composition, but only those four or five spin echo signals which are obtained during an interval derived from the respiration curve. The selection of these four or five scans during the image data pickup has the disadvantage that variations in e.g. the depth of respiration cannot be taken into consideration. Therefore, there is a limitation to the taking of MR scans in the course of the image data pickup always in the same movement phase, as is desirable for a disturbance-free MR image.
It is an object of the present invention to develop a method and a device for MR imaging which will improve the image quality of normally-moving organs, such as the heart and lungs.